1. Field of the Invention
The present invention relates to an imaging apparatus, and more particularly, to an auto-compensating pick mechanism for an imaging apparatus and operating method therefor.
2. Description of the Related Art
A typical image forming apparatus, such as an electrophotographic printer, includes a media sheet supply system having a sheet feed assembly and a supply tray which holds a plurality of print media sheets, such as paper. The media sheets are held in the supply tray until a print job is requested, and ideally are transported one by one to an electrophotographic (EP) assembly within the printer where a latent image is transferred thereto.
One type of sheet feed assembly is an auto compensating sheet feeding assembly. The auto compensating sheet feeding assembly includes a pick roller (or pick rollers) and a gear train which transmits both a rotational force and a downward force to the pick roller. In such an auto compensating sheet feeding assembly, the pick arm is pivoted around its input gear causing a rotation of the pick arm and pick roller to apply increasing pressure by the pick roller to the top sheet until the top sheet is moved.
A supply tray of an image-forming apparatus can be variously configured. For example, one known configuration includes a supply tray having a comer buckler. In a comer buckler system, the pick roller engages the top sheet of the media stack, and transports it toward the comer buckler. The top sheet engages the comer buckler to separate it from the immediately subsequent sheet, and ideally, only it passes the buckler and moves into the paper path.
Another type of known supply tray includes a ramped surface, or dam, at an end thereof which is used to buckle (bend) and separate the top sheet from the immediately subsequent sheet. The pick roller picks the top sheet of the paper stack and moves the top sheet into a paper path having the dam at an end thereof. The moved sheets engage the dam and ideally only the top sheet is separated from the immediately subsequent sheet and passes the dam into the paper path of the printer.
In such media sheet supply systems, however, certain operating situations can occur wherein the auto compensating sheet picker simultaneous picks two or more sheets of media from the media stack. This is commonly referred to as a multi-feed.
Such multi-feeds often result in jamming the paper path of the imaging apparatus.
The auto compensator pick technology is designed to feed a wide range of media weights without requiring adjustments or special trays. This technology is based upon a design that applies only as much normal force to the paper stack as is necessary to overcome the resistance produced by the separating device, such as a comer buckler or a dam. Light weight paper typically requires little normal force, whereas heavyweight paper requires large normal forces. The range of printer speeds with which this technology is used is also large. However, there exists some media whose characteristics fall outside the normal ?high force for heavy, low force for light? rule. In addition, it has been found that as the print speed goes up, the surface velocity control of the sheet picker roller becomes more critical. With traditional velocity control, as the printer speed goes up, the amount of initial normal force goes up as well, primarily due to the amount of back lash in the gear drive train of the system. This results in an increase in multi-feeds.
Backlash occurs to some extent in all gear trains. The more gears in the path from source to destination, the more backlash exists. When the backlash is taken up at a slow speed, the jerk to the roller surface (the change in acceleration per unit time, da/dt) is low. Jerk at the roller surface translates into a spike in normal force to the stack assuming there is no slip from the drive roller to the paper. If the speed of the motor increases quickly, i.e., the acceleration is quick, the jerk to the paper can be large if the driving force has reached its speed before the backlash in the gear train is reached. Thus, the amount of normal force applied to the paper stack is directly proportional to the amount of jerk. Ideally, the normal force applied to the stack should only be enough to offset the force applied by the paper to the roller surface (caused by the paper being stopped by the buckler or dam). Any additional normal force tends to allow sheets to stick together and travel as a pack.
Therefore, a need exists for a method and apparatus for controlling an auto compensator pick mechanism to reduce the occurrence of multi-feeds.